File:The American annual of photography (1914) (14780700505).jpg

Identifier: americanannualof28newy (find matches)
Title: The American annual of photography
Year: 1914 (1910s)
Authors:
Subjects: Photography
Publisher: New York : Tennant and Ward
Contributing Library: Harold B. Lee Library
Digitizing Sponsor: Brigham Young University

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ocal, inasmuch as it is found by dividing thefocal length of the lens by the diameter or opening of thestops. For example, if the stop has a half inch openingand the focus of the lens is eight inches, the stop number willbe known as F/16. This is not mathematically correct withsome types of lenses, but so little out that it will do for allpractical purposes. F numbers usually begin at 4.5 andrun up to 64. The various numbers in the series will be dealtwith further on. U. S. or Uniform System, which is the correct title—not United States, or Uncle Sam, as enthusiastic Ameri-cans are apt to call them, begin at i, then 2, and double eachtime until 256 is reached. It is a handy system, as an expo-sure once known for any particular stop gives readily theproportionate exposure for every other stop. Thus No. 2requires double the exposure of No. i and No. 16, four timesthe exposure given for No. 4, and so on. To show the corresponding numbers of each system, thefollowing will be useful: 276
Text Appearing After Image:
uo o 4> H CO QOO & HO wo Q U. S. No.—I, 2, 4, 8, i6, 32, 64, 128, 256. R No.—4, 5, 6, 8, II, 16, 22, 32, 45, 64. The speed value of all lenses are identical when stoppeddown to the same opening. Thus any lens using stop 8,whether an anastigmat, or only a rectilinear, will require thesame exposure on a given subject. Then again, at F/16both systems are alike, this being a common radiating pointfor each. Stops are used to correct the depth of definition in a pic-ture, as well as influencing the exposure required. For in-stance, we focus our view with the lens wide open, getting theprincipal object clear and distinct, then we reduce the open-ing until we get our other planes sharp. This, of course, in-creases the exposure, but pictorially it is not always a success,as we see in the finished print everything microscopicallysharp, which is different to the human eye, which has toadjust itself to the various planes as we bring them to a focus. On fixed focus cameras the use of st

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